The present invention relates to a lens unit, and more particularly, to a lens unit suitable for use in a compact image capturing device. Further, the present invention relates to an image capturing device.
Most of the image capturing devices that are presently being used are digital cameras, which use charged coupled device (CCD) image sensors or complementary metal oxide semiconductor (CMOS) image sensors as image capturing elements. Thus, such an image capturing device must use a lens unit of which capacities differ from that of a so-called silver salt camera. For example, the sensitivity to diagonal incident light is lower in a CCD image sensor or CMOS image sensor than in a silver salt film. Thus, a CCD image sensor or CMOS image sensor must have a small chief ray angle (CRA), which is the incident angle of light rays entering an image capturing device. Due to the improvements made on the integration of image capturing devices, the image quality must be improved even for image capturing devices used in compact camera or mobile phones. At the same time, such an image capturing device must be reduced in size. Furthermore, an image capturing device that is compact and used in a compact camera or mobile phone must have a structure that lowers manufacturing costs so that relatively inexpensive products can be supplied.
To improve the image quality, aberrations must be lowered. Thus, instead of a lens unit having a three-lens structure, which may easily be designed to be compact, it is desirable that a lens unit having a four-lens structure be used, which more easily suppresses aberrations. Japanese Laid-Open Patent Publication Nos. 2002-228922, 2003-255222, 2005-91666, and 2006-301403 describe compact lens units having four-lens structures.
Japanese Laid-Open Patent Publication Nos. 2002-228922 and 2003-255222 each describe a so-called front stop lens system in which the stop is located further toward an image capturing side (hereinafter referred to as the “object side”) from the lens located at the furthermost object side. In such a structure, it is generally known that when misalignment occurs in a lens located toward an image capturing element side (hereinafter referred to as the “image side”) from the stop, the misalignment greatly affects the image quality. As a result, there is a tendency for product defects to occur due to such misalignment. This lowers the manufacturing yield of the lens unit and increases manufacturing costs.
Japanese Laid-Open Patent Publication Nos. 2005-91666 and 2006-301403 each describe a so-called middle stop lens system in which the stop is located toward the image side from the lens located at the furthermost object side. In such a structure, it is generally known that such a structure increases the CRA. However, the lens unit for a digital camera is strongly required to have a small CRA as described above. Thus, the refractive index of each lens must be adjusted to decrease the CRA. However, when increasing the thickness of each lens or the intervals of the lenses to adjust the refractive index, the entire length of the lens unit increases. This enlarges the lens unit.